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STRUCTURE, AND EMPLACEMENT OF THE BUCK CREEK MAFIC/ULTRAMAFIC COMPLEX, CLAY CO., NC
PETROLOGY AND GEOCHEMISTRY OF METATROCTOLITES IN THE BUCK CREEK ULTRAMAFIC COMPLEX, CLAY COUNTY, NC
PETROGENESIS AND SIGNIFICANCE OF THE BUCK CREEK MAFIC-ULTRAMAFIC COMPLEX, WESTERN NC
V.L. PETERSON (Geosciences, Western Carolina Univ., Cullowhee, NC); M. EMILIO, J.G. RYAN (Dept. Geology, Univ. South Florida, Tampa, FL)
The Buck Creek Mafic-Ultramafic complex (BCUC) is a fragment of oceanic lower crust that has preserved a complex metamorphic history, due both to the varied rock types present and to contrasting rheologies. The complex covers approximately 20 sq. km and is enclosed by schists and paragneisses of the Tallulah Falls Fm. Amphibolite encloses the BC ultramafic body, dominated by dunite with lenses of variably metamorphosed troctolite. Major element compositions of BCUC rocks show a nearly continuous transition from dunite to troctolite and from troctolite to gabbro (amphibolite). Troctolite - dunite and troctolite - amphibolite are interlayered at cm to meter scales, and transitional rocks are common.
Two meta-troctolite lithologies are present. A hard blue metatroctolite preserves anhydrous assemblages formed at ~825 C and ~10 kbar. It is a strong unit that forms map- and outcrop-scale lenses and boudins. The hydrated meta-troctolite (edenite-margarite schist (EMS)) contains a primary assemblage of edenitic amphibole + anorthite +/- corundum +/- spinel +/- kyanite +/- zoisite with retrograde margarite. Assemblages in the EMS rocks indicate peak P-T conditions of >11 kbar and 700-850 C. EMS, dunites and amphibolites on the western side of the complex, near the Chunky Gal Fault, preserve a prominent retrograde signature of ~525 C and 4-6 kbar. Hydration appears to be tied to phases of deformation. Hydrated troctolite is most voluminous in boudin necks, the hinge regions of map-scale folds, and in shear zones, all regions where structural features may have favored local fluid infiltration. Relating structural features to anhydrous and hydrated assemblages improves constraints on the P-T history of BCUC. [Back to top]
STRUCTURE, AND EMPLACEMENT OF THE BUCK CREEK MAFIC/ULTRAMAFIC COMPLEX, CLAY CO., NC
MCCOY A., RAHL J., COCHRANE D., KATES K., LEE A., LINDENBERG M., LIZEE T., MEYERS S., MORMAN S., OLESKY M., PAULEY T., THOMAS C. (1998 REU Program); V.L. PETERSON (Geosciences, Western Carolina Univ., Cullowhee, NC); M. EMILIO, J.G. RYAN (Dept. Geology, Univ. South Florida, Tampa, FL 33620
The dunite bodies of the Buck Creek mafic-ultramafic complex (BC) enclose metatroctolite lenses + exposures of anothosite and clinopyroxenite, and are enclosed within hbld amphibolites (metagabbro). Strong interlayering and local gradation between units, along with geochemistry, indicate BC is a layered mafic igneous sequence. Erosion-resistant, anhydrous metatroctolite (herein troctolite) lenses are separated from dunite by hydrated edenite-margarite schists (EMS) and/or actinolite-chlorite schists (ACS: talc-chlorite unit of Hadley(1949)), both of troctolite protolith. Outcrop-scale, folded troctolite boudins, enveloped in EMS, point to early pull-apart of troctolite layers, assisted by contact hydration, to explain the discontinuous character of the unit. Troctolite caps hilltops and was not found in the low-lying SE region of the complex, as mapped by Hadley (1949). Gradations in the field between hydrated metatroctolite and amphibolite assemblages, corroborated geochemically, suggest that all units were originally part of the same complex. Localized shear zones, discontinuities along the mafic-ultramafic contact, and the presence of a thin mylonite zone along the eastern Tallulah Falls Fm.- amphibolite contact indicate ductile remobilization along some contacts, perhaps correlated with movement of the local Chunky Gal Fault. Trondhjemite dikes appear to crosscut BC units.
Relict mineralogical layering is seen in troctolites and some EMS. Well-developed foliations (S1) are found in amphibolites, ACS, EMS, and locally in troctolite. In northwest BC, the dominant foliation, slightly oblique to relict compositional layers and consistent with minor disharmonic folds in EMS, indicates the limb of a plunging-inclined fold with a NW-dipping axial surface. S1 foliation is folded by map-scale, northerly-plunging upright (F2) folds. The dunite map pattern may reflect Ramsay Type 2 fold interference.
Field observations are consistent with a model of BC as an oceanic crustal fragment emplaced during collision. Peak metamorphic assemblages (Tenthorney et al., 1996; Emilio, 1998) indicate burial depths of 30-40 km. Possible emplacement mechanisms include detached-slab subduction (Davies and Blanckenburg, 1995) with imbrication on the footwall of the Hayesville Thrust Fault, or incorporation into a tectonic melange. Trondhjemites may be residual melts from migmatized sediments in the melange. [Back to top]
PETROLOGY AND GEOCHEMISTRY OF METATROCTOLITES IN THE BUCK CREEK ULTRAMAFIC COMPLEX, CLAY COUNTY, NC
THOMAS C., LEE A., OLESKY M.,COCHRANE D., KATES K., LINDENBERG M., LIZEE T., MCCOY A., MEYERS S., MORMAN S., RAHL J., PAULEY T.(1998 REU Program); V.L. PETERSON (Geosciences, Western Carolina Univ., Cullowhee, NC); M.EMILIO, J.G. RYAN (Dept. Geology, Univ. South Florida, Tampa, FL).
The Buck Creek mafic-ultramafic complex (BC) includes several different rock types with troctolitic protoliths. "Blue" coronal metatroctolites and corundum-bearing edenite-margarite schists (EMS) form lenticular pods and are found both within the dunite and interlayered between dunite and amphibolite. A 2 km sinuous band of actinolite-chlorite schist (ACS) (talc-chlorite schist of Hadley (1949)) lies within amphibolite in the northern portion of BC. ACS grades into interlayered troctolite and dunite to the east. The derivation of ACS from a troctolite-dunite protolith is indicated by remnant olivine + chromite in some samples. Deformation lamellae are found in olivine, but no other primary igneous minerals survive. Chlorite + actinolite are the dominant ACS phases, with lesser serpentine ± carbonate ± talc ± iddingsite. Formation of ACS involved hydration and carbonation; weak fabric development in ACS samples indicates hydration/carbonation was late in the tectonic sequence.
Geochemically, troctolite, EMS and ACS form a single array, varying similarly in MgO ((6-35 wt%) and Al2O3 (7-29 wt.%), and averaging 43% SiO2. ACS samples fall toward the MgO-rich end of this array, and grade into dunite compositions. On plots of MgO v. Al2O3, ACS- EMS-troctolite samples are co-linear with relict An88 plagioclase and Fo89 olivine found in the coronal troctolites, suggesting that most of these rocks began as mixtures of accumulating olivine and plagioclase crystals.
BC metatroctolites indicate a high-temperature metamorphic progression from (a) complex corona textures composed mostly of anhydrous Opx+Cpx+spinel, to (b) partial hydration of coronal assemblages, producing amphibole + coarse corundum ± sapphirine, to (c) the development of foliated textures and a thoroughly hydrated assemblage of edenitic amphibole+plagioclase+margarite ± chlorite+fine, disseminated corundum. (b) and (c) may represent high-T¡ retrograde alteration due to localized fluid influx and/or deformation. ACS samples, however preserve no clear mineralogical evidence for the higher P-T metamorphism experienced by the other BC metatroctolite lithologies.[Back to top]
MORMAN S., COCHRANE D., KATES K., OLESKY M., PAULEY T., THOMAS C., LEE A., LINDENBERG M., LIZEE T., MCCOY A., MEYERS S., , RAHL J., (1998 REU Program); I. SAVOV, J.G.RYAN (Dept. Geology, Univ. South Florida, Tampa, FL. 33620); V.L. PETERSON (Geosciences, Western Carolina Univ., Cullowhee,NC)
Amphibolite samples collected from the Buck Creek mafic-ultramafic complex during the 1997-1998 REU program field seasons show distinct petrographic and geochemical patterns. All samples contain hornblende+plagioclase±epidote±scapolite with accessory quartz, titanite, clinopyroxene, apatite, ilmenite, and rutile, and display variably foliated poikiloblastic textures. Relative hornblende-plagioclase abundances vary from <20% to ~90% plagioclase, with the plagioclase rich samples found near the band of actinolite-chlorite schist (ACS) in the north. An contents in amphibolites near contacts with meta-troctolite units (including ACS) are higher (An35 - 44) than those near the margins of the complex (~An26). Scapolite replaces plagioclase, and epidote replaces hornblende in many samples. Sphene rims ilmenite, locally with symplectic intergrowths. The Type 1/Type 2 distinctions of MacElhaney and McSween (1981) are unclear in our samples, as epidote and sphene were evident in all samples. However, most scapolite and clinopyroxene occur in samples from the north and west.
Geochemically, amphibolites form two distinct groups on plots of MgO vs. Al2O3, Ni vs Ti/Cr, and Ti vs. Sr. Sr and Ni contents of samples from the south are higher (Ni>200ppm and Sr>180ppm) and more variable than those from the north and west. High Ni and Sr correlate with high and variable MgO and Al2O3, consistent with cumulate gabbro protoliths. High and variable TiO2 in these "cumulate" samples may indicate the original crystallization of a Ti-rich phase (ilmenite?). The low Ni and Sr amphibolites overlap fields for MORB and ocean floor basalts on a variety of discrimination diagrams, suggesting these rocks may represent original magma compositions. Our amphibolites are chemically consistent with origins as cumulates and diabase/gabbros from a high-Ti (oceanic) ophiolitic succession, as defined by Serri (1981). [Back to top]
PETROGENESIS AND SIGNIFICANCE OF THE BUCK CREEK MAFIC-ULTRAMAFIC COMPLEX, WESTERN NC
RYAN, J. G. (Dept. Geology, Univ. South Florida, 4202 E. Fowler Ave. Tampa, FL 33620); PETERSON, V.L. (Geosciences, Western Carolina Univ. Cullowhee, NC); 1997-98 BUCK CREEK REU PROGRAM PARTICIPANTS
Intensive field and geochemical studies of the Buck Creek mafic-ultramafic complex (BC) as part of our 1997-98 REU program has given us a detailed petrologic and geochemical picture of this massif. The complex includes five mappable lithologies: dunite, metatroctolite, edenite-margarite schist (EMS), actinolite-chlorite schist (ACS), and amphibolite; along with isolated occurrences of clinopyroxenite and meta-anorthosite. These lithologies are finely interleaved, in some places on the scale of meters. Early disaggregation of troctolite layers due to significant competency contrasts, was followed by large-scale inclined to recumbent folding associated with the dominant S1 foliation and subsequent formation of upright F2 folds. Local ductile shearing and remobilization along contacts likely post-dated formation of S1 fabrics.
Lithologic distinctions in BC blur when the units are examined geochemically. Meta-troctolites, EMS and ACS appear largely to be variably hydrated and metamorphosed variants of an Ol+Plag cumulate protolith, while many amphibolites and some EMS samples bear strong chemical similarities to cumulate gabbros. A subset of the amphi-bolites show bulk chemical and immobile trace element systematics similar to MORB and/or back-arc basin basalts. Thus, despite profound, multistage metamorphism (peak conditions ~800¡C, ~12 kbars: Emilio et al. 1998), BC preserves the chemical fingerprints of an ophiolite sequence, and may thus represent a fragment of ancient ocean crust.
Lithologic, chemical, REE, and isotopic comparisons suggest BC bears similarities to the Lake Chatuge mafic-ultramafic complex (LC) to the southwest, but is markedly different from the Webster-Addie ultramafic complex, and other smaller units to the northeast. The high peak metamorphic grade preserved in both BC and LC point to very early (pre-Taconic) origins. BC and LC may thus represent a deep sliver of ocean crust emplaced during the earliest Blue Ridge collisional event. Later deformation during subsequent convergence/collisions diaggregated this sliver and thrust BC into overlying forearc metasedimentary rocks (Tallulah Falls/Ashe Fm).[Back to top]
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